1. Field of the Invention
This invention relates to a radiation image read-out apparatus which reads out a radiation image stored on a stimulable phosphor sheet.
2. Description of the Related Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, xcex1-rays, xcex2-rays, xcex3-rays, cathode rays or ultraviolet rays, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light or a laser beam, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. There has been known a radiation image read-out apparatus in which a stimulating light beam such as a laser beam is caused to scan a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) which has been exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet so that the stimulable phosphor sheet emits stimulated emission, the stimulated emission is detected by the use of a light collecting mirror or an optical guide, the detected stimulated emission is photoelectrically converted by a photomultiplier into an image signal (a radiation image signal), and then the stimulable phosphor sheet is exposed to erasing light after the image signal is obtained from the stimulable phosphor sheet so that the residual energy of the radiation is fully released from the stimulable phosphor sheet.
The radiation image signal thus obtained is subjected to image processing such as gradation processing and/or frequency processing and a radiation image of the object is reproduced as a visible image on the basis of the processed radiation image signal on a recording medium such as a photographic film or a display such as a CRT.
In conventional radiation image read-out apparatuses, there has been a problem that a scanning optical system which can cause a stimulating light beam to scan the stimulable phosphor sheet at least over a half width of the stimulable phosphor sheet and a light condensing/detecting system which can detect stimulated emission diverging at least over a half width of the stimulable phosphor sheet are required, which makes it difficult to miniaturize the apparatus.
In order to overcome the problem, there have been proposed systems in which a line source such as an LED array or an EL element is used as the stimulating light source and a semiconductor sensor is used as an element for detecting the stimulated emission. However such systems have not been put into practice because of poor intensity of the stimulating light source and/or a poor SN ratio of the semiconductor sensor including the processing circuit.
Further, in order to improve the durability of the stimulable phosphor sheets and to improve the quality of the image, there has been proposed a rigid stimulable phosphor sheet. The conventional radiation image recording and reproducing apparatuses are generally provided with a sub-scanning system which conveys the stimulable phosphor sheet in a direction substantially normal to the direction in which the stimulating light beam scans the stimulable phosphor sheet. Use of the rigid stimulable phosphor sheet requires the sub-scanning system to be at least twice as long as the length of the stimulable phosphor sheet, which is disadvantageous in that the overall size of the apparatus becomes very large. Further, when the sub-scanning system is arranged to convey the read-out system including the scanning optical system and the light condensing/detecting system in the sub-scanning direction, the sub-scanning system should be very firm since the read-out system is very large and a very large space is required to move the large read-out system, which adds to the cost and increases the overall size of the apparatus.
Further, when the radiation image read-out apparatus is of a point scan system in which the stimulable phosphor sheet is exposed to the stimulating light beam from pixel to pixel, the read-out time cannot be sufficiently shortened due to the response time of the stimulable phosphor sheet.
In view of the foregoing observations and the description, the primary object of the present invention is to provide a radiation image read-out apparatus which can be small in size.
Another object of the present invention is to provide a radiation image read-out apparatus which can read out a radiation image at a high speed.
In accordance with the present invention, there is provided a radiation image read-out apparatus in which a stimulable phosphor sheet storing thereon a radiation image is exposed to stimulating light and stimulated emission emitted from the stimulable phosphor sheet upon stimulation by the stimulating light is detected and an image signal representing the radiation image stored on the stimulable phosphor sheet is obtained by photoelectrically converting the stimulated emission, the radiation image read-out apparatus characterized by having
a read-out system comprising an array of a plurality of read-out heads, each comprising a stimulating light source which projects the stimulating light onto the stimulable phosphor sheet and a photodetector which detects the stimulated emission, and
a conveyor system which conveys the stimulable phosphor sheet and the read-out system relative to each other in a direction intersecting the direction of the array of the read-out heads.
The conveyor system may move one of the stimulable phosphor sheet and the read-out system or both of them, though it is preferred from the viewpoint of reducing the overall size of the radiation image read-out apparatus that the conveyor system moves the read-out system with the stimulable phosphor sheet kept stationary.
In the read-out system, the read-out heads may be arranged either linearly or two-dimensionally. That the read-out heads are linearly arranged means that the read-out heads are arranged in a row so that the read-out scanning lines of the respective read-out heads (the lines along which the stimulating light beams are caused to scan the stimulable phosphor sheet in the respective read-out heads) are substantially aligned with each other on the stimulable phosphor sheet, and that the read-out heads are two-dimensionally arranged means that the read-out heads are arranged in a plurality of rows so that the read-out scanning lines of the respective read-out heads form a plurality of substantially parallel lines on the stimulable phosphor sheet. In the latter case, the read-out heads may be arranged so that the read-out scanning lines of the respective read-out heads are arranged in a zigzag pattern on the stimulable phosphor sheet.
The xe2x80x9cdirection of the array of the read-out headsxe2x80x9d means the direction in which the row or the rows of the read-out heads extends, and xe2x80x9ca direction intersecting the direction of the array of the read-out headsxe2x80x9d means any direction which intersects the direction of the array of the read-out heads without limited to the direction normal to the direction of the array of the read-out heads.
The read-out system may include at least two read-out heads so long as the sum of the read-out widths (the width over which the read-out head reads out an image signal) of the read-out heads is substantially equal to the necessary overall read-out width (generally about 350 mm or 430 mm). It is preferred that the read-out width of each read-out head be about xc2xd to {fraction (1/10)} of the necessary overall read-out width.
It is preferred that the read-out heads simultaneously read out image signals, though it is not necessary and the read-out heads may read out image signals in sequence.
It is preferred that the stimulating light source of each read-out head comprises a semiconductor laser and an optical deflector. As the optical deflector, various deflectors including a miniaturized polygonal mirror, a micro scanner, an acousto-optic deflector, and an electro-optic deflector and the like may be employed. Especially, the acousto-optic deflector and the electro-optic deflector are preferable when a plurality of read-out heads simultaneously read out image signals since they are easy to control the deflecting direction. In this case, it is possible to cause the read-out heads to read out image signal in synchronization with each other by controlling the optical deflectors of the respective read-out heads so that the deflecting directions of the respective optical deflectors become substantially equal to each other.
It is preferred that the photodetector comprises a photoelectric convertor element and a stimulating light cut filter which cuts light of a wavelength near the wavelength of the stimulating light.
Further, it is preferred that the read-out system be provided with an erasing light source for erasing a radiation image stored on the stimulable phosphor sheet.
Further, it is preferred that the read-out system be provided with a read-out noise suppressing means which suppresses image read-out noise due to optical crosstalk between adjacent read-out heads.
The read-out noise suppressing means may comprise, for instance, a light-shielding member which optically prevents optical crosstalk between adjacent read-out heads, or a correction means which corrects an image signal to compensate for influence of the optical crosstalk on the image signal, or a read-out control means which controls the read-out heads so that adjacent read-out heads do not simultaneously read image signals.
Further, it is preferred that the read-out system be provided with a spacer member which keeps constant the distance between the read-out heads and the stimulable phosphor sheet. It is especially preferred that the spacer member is in the form of a revolvable roller.
Since being provided with a read-out system comprising an array of a plurality of read-out heads, each comprising a stimulating light source which projects the stimulating light onto the stimulable phosphor sheet and a photodetector which detects the stimulated emission, the radiation image read-out apparatus in accordance with the present invention may be smaller in the scanning distance than the conventional radiation image read-out apparatus which is provided with a single stimulating light source. Accordingly, in accordance with the present invention, the read-out system can be small in size, which makes it feasible to reduce the overall size of the radiation image read-out apparatus.
Further, when the conveyor system moves the read-out system with the stimulable phosphor sheet kept stationary, the necessary space can be smaller as compared with when the stimulable phosphor sheet is moved. Further, since the scanning optical system of each read-out head is smaller than the scanning optical system in the conventional radiation image read-out apparatus, the conveyor system need not be so firm and can be manufactured at low cost.
Further, since having a plurality of read-out heads which can be simultaneously operated, the radiation image read-out apparatus in accordance with the present invention can read an image at a higher speed than the conventional radiation image read-out apparatus of a point scan system.